A loop of thread weighing 0.125th of a grain
caused a petiole to curve; but the stimulus was not sufficient, the
loop remaining suspended, to cause a permanent flexure. If a much
heavier loop be placed in the angle between the petiole and the stem,
it produces no effect; whereas we have seen with Clematis montana
that the angle between the stem and petiole is sensitive.
Tropaeolum peregrinum.--The first-formed internodes of a young plant
did not revolve, resembling in this respect those of a twining plant.
In an older plant the four upper internodes made three irregular
revolutions, in a course opposed to the sun, at an average rate of 1
hr. 48 min. It is remarkable that the average rate of revolution
(taken, however, but from few observations) is very nearly the same
in this and the two last species, namely, 1 hr. 47 m., 1 hr. 46 m.,
and 1 hr. 48 m. The present species cannot twine spirally, which
seems mainly due to the rigidity of the stem. In a very young plant,
which did not revolve, the petioles were not sensitive. In older
plants the petioles of quite young leaves, and of leaves as much as
an inch and a quarter in diameter, are sensitive.
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